1. Field of the Invention
The present invention is directed to the preparation of dendritic macromolecules having controlled surface functionality by a novel convergent approach. Once prepared, the dendritic molecules are used to prepare new macromolecular assemblies with unusual architectures and properties.
2. Description of the Prior Art
Interest in dendritic molecules dates back to the early fifties with the publication of a theoretical paper by Flory, J., Am. Chem. Soc., 74, 2719(1952) stating, "Highly branched polymer molecules may be synthesized without incidence of gelation through the use of monomers having one functional group of one kind and two or more of another capable of reacting with the former." Few examples of purposeful attempts to control molecular constitution through this approach were investigated until the late 1970's when Vogtle and coworkers, Synthesis 155 (1978), described a "cascade" approach to branched oligomeric products through a Michael-type addition of a polyfunctional amine to acrylonitrile followed by reduction of the newly formed nitrile chain ends to yield a next generation of reactive amine groups.
Although interesting, this approach has not been extended beyond a product having a molecular weight of 790 Daltons. A subsequent attempt, described in U.S. Pat. No. 4,289,872, involved a stepwise protection-deprotection approach for the condensation of lysine into highly branched high molecular weight products for which little characterization data was made available. More recently, Newkome used a nucleophilic displacement reaction on a multifunctional core to produce, after two stages of reaction, a cascade molecule coined "arborol" with molecular weights of up to 1600. See, for example, Aharoni et al, Macromolecules 15, 1093 (1982); J. Org. Chem. 50, 2004 (1985); Newkome et al, J. Chem. Soc. Chem. Commun., 752 (1986); and Newkome et al, J. Am. Chem. Soc. 108, 849 (1986).
The most extensive published studies of dendritic molecules are directed to the "starburst" polymers. See, for example, U.S. Pat. Nos. 4,507,466; 4,558,120; 4,568,737; 4,587,329; and 4,737,550. Such "starburst" polymers are produced by allowing a polyfunctional amine core molecule to react with excess methyl acrylate in a Michael-type addition. Each arm of the resulting star-branched molecule is then reactivated to an amineterminated moiety by exhaustive amidation using excess 1,2-diaminoethane to afford a chain extended product in which each primary amino group becomes a new branch point in the next series of Michael additions.
"Starburst" polymers may also be made using pentaerythritol or an analogous triol as the core moiety from which a highly branched polyether starburst polymer may be built in successive deprotection-alkylation steps.
Kim et al, Polymer Preprints, 29 (2), 310 (1988) describe the synthesis of a hyperbranched polyphenylene with a molecular weight of up to 4,000 by aryl-coupling reaction. In this procedure, growth is irregular, affording a product mixture with high polydispersity.
In all but the last of these approaches for producing dendritic molecules, a polyfunctional reactive core was used to initiate dendritic growth and the radially grown interior layers carried on their outer surface a very large number of reactive functionalities. This is referred to as a "divergent" approach for producing dendritic molecules. The use of such highly functionalized cores provides for extremely efficient growth. For example, from a tetrafunctional core and tetrafunctional monomer, Hall and Tomalia, J. Organic Chemistry, 52, 5305 (1987) prepared in only three iterations ("generations") polyethers containing nominally 108 surface functional groups. However, such a high packing density appears to prevent further regular growth. In fact, with all systems in which growth requires the reaction of large numbers of surface functional groups, it is difficult to ensure that all will react at each growth step. This poses a significant problem in the synthesis of regular monodispersed and highly organized structures since unreacted species may lead to failure sequences or spurious reactivity at later stages of the stepwise growth sequence.
Although great strides have been made in the control of polymer structures through the development of new living polymerization or the use of macromonomers or telechelic polymers, it is still very difficult to control the shape of macromoleculer assemblies and the spatial placement of reactive groups within their confines. In view of the high degree of order that such highly branched structures may exhibit, and the number of potentially interesting and novel molecular topologies that may be envisioned, the control of their surface functionality both in terms of the number and the placement of surface functional groups is an important target.